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Claim and Claim Adjustment Expense Reserves - Additional Information (Detail) - CNA Financial [Member] - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Dec. 31, 2010
Dec. 31, 2018
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Catastrophe losses, net of reinsurance $ 58 $ 34    
Net A&EP adverse development before consideration of LPT   113    
Fair value of the collateral trust account 3,000     $ 2,700
Asbestos and Environmental Pollution Reserves [Member]        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Net A&EP claim and allocated claim adjustment expense reserves     $ 1,600  
Aggregate limit under A&EP Loss Portfolio Transfer     4,000  
Ceded A&EP claim and allocated claim adjustment expense reserves     1,200  
Reinsurance premium paid to NICO under A&EP Loss Portfolio Transfer     2,000  
Net reinsurance receivables transferred to NICO under A&EP Loss Portfolio Transfer     215  
Total consideration     $ 2,200  
Cumulative amounts ceded under the Loss Portfolio Transfer 3,100     3,100
Remaining unrecognized retroactive reinsurance benefit 352     $ 374
Property and Casualty Operations [Member]        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Favorable net prior year development $ 14 $ 39